if it has to be, give away some provinces to other factions you are allied with or who are neutral to you but at war with your enemy to build a buffer zone. it may be necessary to give them a bit of money to make them take the provinces (one of many stupidities concerning the RTW-diplomacy-engine: you have to pay for the acceptance of land given away!!! ). but it can be a good deal for you anyhow because it lessens the pressure to your borders so you need not so many troops and do not have to pay for retraining this often. and friendly neighbours will also trade with you, enlarging your treasury.